To components which require soldering, for example, components for electronic devices such as chip components, crystal oscillators, bumps, connectors, lead frames, hoops, semiconductor packages and printed boards, tin-lead alloy plating has been applied conventionally. In the fabrication or the like of printed boards, tin-lead alloy plating films have been widely used as etching resists.
In recent years, however, tighter regulations have been introduced on the use of lead as a measure for environmental protection, leading to an increasing desire for lead-free plating as a substitute for tin-lead alloy plating materials. With respect to lead-free tin alloy plating as one type of such lead-free plating, a variety of developments are being carried out. Examples of lead free plating include tin plating, tin-copper alloy plating, tin-silver alloy plating, tin-bismuth alloy plating, and the like. However, conventional tin plating films are known to be prone to the formation of whisker-shaped crystals called “whiskers”, and these whiskers develop a problem such as short-circuiting. Further, lead-free tin alloy plating films developed to date are also still insufficient although compared with tin plating films, inhibitory effects are observed for the formation of whiskers.
Even if lead-free tin alloy plating is effective for the formation of whiskers, it tends to require irksome control of its plating bath because it is a type of alloy plating and needs to control two or more metal elements. Especially in a tin-silver alloy plating bath or tin-bismuth alloy plating bath, a substantial difference exists in potential between the two metal elements so that, if a tin anode surface or a plated workpiece is left immersed in the plating bath while no current is applied, silver or bismuth is allowed to replace and deposit on its surface and the tin anode or workpiece may become useless.
For the inhibition of the formation of whiskers, the following methods have been conventionally used (see Mitsubishi Electric Corporation Technical Report, vol. 53, No. 11, 1979 (Non-patent Document 1)), but they are accompanied by problems, respectively.    (1) Nickel plating is performed on an undercoat of tin or tin alloy plating: A nickel plating film acts as a barrier against the formation of an intermetallic compound between copper as a substrate and tin as a plating film to inhibit the formation of whiskers. There are, however, numerous components which do not permit nickel plating due to characteristics required for them.    (2) Tin or tin alloy plating is applied at a greater film thickness (10 to 20 μm or greater): An increase in film thickness inhibits the formation of whiskers because effects of an internal stress produced by the formation of an intermetallic compound do not reach the surface. There are, however, many electronic devices which do not permit an increase in plating film thickness.    (3) Application of heat treatment and reflow treatment after tin or tin alloy plating: Application of heat treatment and reflow treatment after tin or tin alloy plating makes it possible to form a layer of a stable intermetallic compound (Cu3Sn or the like) beforehand, and moreover, to relax an internal stress of a plating film and to inhibit the formation of whiskers. As a result of the heat treatment and reflow treatment, however, an oxide film is formed on the tin plating film to result in deteriorated solderability.
Patent Document 1:                Japanese Patent Laid-open No. 2003-293185        
Patent Document 2:                Japanese Patent Laid-open No. 2005-2368        
Non-patent Document 1:                Mitsubishi Electric Corporation Technical Report, Vol. 53, No. 11, 1979        